This invention relates to steam turbines generally and in particular to a system and method for decreasing the amount of time required for start-up of a steam turbine during temporary shutdown of the turbine.
In recent years there has been a general decrease in electrical demand in this country by utilities, in particular, which have large steam driven turbines which were designed for constant use. Because of the decrease in electrical demand combined with escalating fuel costs, utilities have been cycling their plants, that is, the plants are selectively shut down for a period of time and then returned to service. A method for the shutdown and restarting of a combined power plant is disclosed in U.S. Pat. No. 4,282,708 issued to Tetsuzo Kuribayashi et al. The Kuribayashi patent discloses a method to supply heated steam to gland sections of a steam turbine in a combined plant, i.e., one having both a steam and a gas turbine, to reduce restarting time of the steam turbine.
Large turbines require a long warm-up period each time they are shut down and allowed to cool off. Generally, present practice during start-up is to run the turbine at low speed for a period of time until critical areas of the machine such as the rotating shaft reach minimum temperatures. For example, if the turbine rotating shaft temperature falls below 250.degree. F., cold-start warming procedure is normally used for restarting the turbine system. This procedure generally includes (1) several hours of applying steam to the turbine to reach the prescribed minimum operating temperature and (2) several additional hours of part speed operation after the turbine has reached the prescribed minimum temperature. In both cases the boiler must be fired at a reasonable rate to achieve minimum steam quality. In a 500 to 750 megawatt system the hourly fuel cost during the warming procedure is approximately $4,000, and typically it takes approximately 4 to 6 hours to achieve minimum operating temperature and an additional 6 to 10 hours of running at part speed warming. From this, it is apparent that the fuel cost alone for restarting the turbine is quite substantial, not to mention other costs such as labor.